<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <h3>message2</h3>
  <script>

    window.addEventListener('message', handleMessage)

    function handleMessage(e) {
      document.querySelector('h3').innerHTML = e.data
      e.source.postMessage('我是message2中的消息', 'http://localhost:3000')
    }

  </script>
</body>

</html>